Electrode potential dependence

Thus, the temperature coefficient of Galvanic potential of an individual electrode can be neither measured nor calculated. Measured values of the temperature coefficients of electrode potentials depend on the reference electrode employed. For this reason a special scale is used for the temperature coefficients of electrode potential It is assumed as a convention that the temperature coefficient of potential of the standard hydrogen electrode is zero in other words, it is assumed that the value of Hj) is zero at all temperatures. By measuring the EMF under isothermal conditions we actually compare the temperature coefficient of potential of other electrodes with that of the standard hydrogen electrode. 

Anderson AB, Albu TV. 2000. Catal3ftic effect of platinum on oxygen reduction. An ab initio model including electrode potential dependence. J Electrochem Soc 147 4229-4238. 

Anderson AB, Neshev NM. 2002. Mechanism for the electro-oxidation of carbon monoxide on platinum, including electrode potential dependence—Theoretical determination. J Electrochem Soc 149 E383-E388. 

The electrode potential depends on time according to the same relationships as in the preceding section. 

Although the accuracy of this explanation will be discussed later, it is easily understood that the behavior of the electrode is greatly influenced not only by the instantaneous potential of the electrode (potential dependence ) but also by the history of the electrode (time dependence). As this example shows, the electrochemical oxidation of methanol is a series of reactions in which methanol, water, intermediates and surface adsorbates are interacting with each other in various ways, and are yet to be fully understood. 

Fig. 4 The incommensurability versus electrode potential dependence for adsorption of Br on Au(lOO). Insets atomic models of Br adlayers corresponding to the potentials below and above the critical point of phase transition [59].

Let us divide the parameters into three terms (1) one representing chemical interactions of the ion and the water molecules that is independent of the charge or potential of the electrode, AG°h (2) a charge or electrode potential-dependent term for the ion and the solvent molecules, AGE and (3) a lateral interaction term, AGt. 

When a biochemical half-reaction involves the production or consumption of hydrogen ions, the electrode potential depends on the pH. When reactants are weak acids or bases, the pH dependence may be complicated, but this dependence can be calculated if the pKs of both the oxidized and reduced reactants are known. Standard apparent reduction potentials E ° have been determined for a number of oxidation-reduction reactions of biochemical interest at various pH values, but the E ° values for many more biochemical reactions can be calculated from ArG ° values of reactants from the measured apparent equilibrium constants K. Some biochemical redox reactions can be studied potentiometrically, but often reversibility cannot be obtained. Therefore a great deal of the information on reduction potentials in this chapter has come from measurements of apparent equilibrium constants. 

The numerical value of an electrode potential depends on the nature of the particular chemicals, the temperature, and on the concentrations of the various members of the couple. For the purposes of reference, half-cell potentials are taken at the standard states of all chemicals. Standard state is defined as 1 atm pressure of each gas (the difference between 1 bar and 1 atm is insignificant for the purposes of this chapter), the pure substance of each liquid or solid, and 1 molar concentrations for every nongaseous solute appearing in the balanced half-cell reaction. Reference potentials determined with these parameters are called standard electrode potentials and, since they are represented as reduction reactions (Table 19-1), they are more often than not referred to as standard reduction potentials (E°). E° is also used to represent the standard potential, calculated from the standard reduction potentials, for the whole cell. Some values in Table 19-1 may not be in complete agreement with some sources, but are used for the calculations in this book. 

Choice D results from the incorrect assumption that electrode potentials depend on the amount of material present. The balanced net reaction for the cell is ... 

Figure 3.3.10 (A) The electrode potential dependence of the Gibbs free energy reaction pathway of the ORR. While the overall reaction has elementary steps that are energetically uphill at +1.23 V (red pathway), all elementary steps become downhill at +0.81 V (yellow pathway) (i.e. at an overpotential of approximately -0.42 V. At this point, the reaction is not limited by kinetics anymore. (B) The experimentally observed current-potential (j-E) relation of the ORR is consistent with the computational conclusions from (A) between +1.23 V and +0.81 V the j-E curve shows an exponential behavior, while at electrode potentials below +0.81 V, the ORR reaction rate becomes oxygen mass-transport limited, which is reflected by a flat ( j-E) profile. Figure adapted with permission from [19]. 

Weston normal element (cell) — Electrochemical -> standard cell showing a particularly stable and reproducible cell voltage. In the international Weston normal element a cadmium amalgam (cadmium content in the solid phase approx. 15 wt %, in the liquid phase approx. 5wt%, total average 12 to 12.5 wt%, the electrode potential depends only on the temperature, not on the mass ratio of liquid and solid phases) and a mercury electrode (half-cell) are combined according to... 

Figure 22 Electrode potential dependence of shifts for CO (circles) and CN (squares) chemisorbed on a 10-nm Pt electrocatalyst in an electrochemical environment. (From Refs. 3 and 4.)... 

FIGURE 1.13. The influence of monoenergetic surface states on Mott-Schottky plot with Vn, shifted to a higher or lower electrode potential depending on the charge on the states, after Morrison."... 

The oxidation state of the spontaneously deposited metal nanoislands is an important issue for interpreting the electrocatalytic activity of the bimetallic electrodes. For that, in situ X-ray photoelectron spectroscopy (XPS) was used to determine the electrode potential-dependent oxidation state of Ru28 and Os29 nanoislands spontaneously deposited on Pt(lll). XPS data concerning the oxidation state of spontaneously deposited Pt on Au(l 11)23 and Ru and Ir on Ru(0001) were reported.30... 

STM tip that was held at fixed distance (10-150 nm.) from the electrode. With time Cu deposit reached the substrate forming a nanowire between the tip and the substrate. They found that the diameter of the wire was of a few atoms and that such a structure has an electrode potential dependent quantized conductance. 

Figure 7-25. Measurements of the single molecule conductance of 6V6 molecular wires performed under electrochemical control in 0.1 M phosphate buffer solution. The electrode potential dependence of the conductance at constant tip-sample bias (Ut = 0.2 V) of a single 6V6 molecule is shown. " ...

It is impossible to measure directly the electrode potentials. Only the electromotive force (emf) of a voltaic cell arising from a combination of two electrodes can be directly measured, which is given as the arithmetical sum or difference of the two electrode potential depending upon their signs. If one of the electrode potential be accurately measured, that of the other may be calculated. The reference electrode arbitrarily chosen for this purpose is the standard hydrogen electrode. Hydrogen gas at 1 atm. pressure and at a temperature of 25°C is slowly bubbled over a platinised platinum electrode which is immersed in a solution of hydrogen ions of unit activity. By convention potential of the half cell reaction... 

The numerical value of an electrode potential depends on the nature of the particular chemicals, on the temperature, and on the concentrations of the various members of the couple. For purposes of reference, half-cell potentials are tabulated for standard states of all the chemicals, defined as 1 atm... 

This equation has the form of the Riccati differential equation with electrode potential dependent coefficients (see Appendix) ... 

Because electrode potential depends on ion concentrations, it is possible to construct a cell from two half-cells composed of the same material but differing in ion concentrations. Such a cell is called a concentration cell. 

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